For the people already owning a bronze ring or planing to buy one.
Bronze is an alloy from copper, tin and rates of brass. Is totally normal for that kind of material to tarnish black when worn on skin for a longer time. Is a reaction between the metal and the human sweat.
This fact is familiar but I want to mention in anyway.
For clean the material is possible to use commercial detergents and polish.
Is also possible for a goldsmith to purify the rings with silver, gold, white gold or rhodium plating. I can also do this on request and with payment in advance.
You asked for a cheaper possibility to produce rings in high numbers because of the increasing silver price. It is still possible to order silver rings but we need a minimum quantity of 500 grams silver before the manufacturer starts the production - so the time for delivery could be very long (8 - 10 weeks) because we first need to gather orders.